Method and apparatus for detection and treatment of tachycardia and fibrillation

ABSTRACT

An automatic implantable device for detecting and differentiating between tachyarrhythmias in order to therapeutically stimulate the heart in response thereto, particularly for distinguishing fibrillation from tachycardia and to provide appropriate therapies for each condition. The event intervals between successive heart depolarizations are measured, stored and classified as within fibrillation or tachycardia interval ranges. The numbers of intervals falling within the fibrillation and tachycardia interval ranges are employed to distinguish fibrillation from tachycardia. The number of intervals required to detect and discriminate between tachycardia and fibrillation in situations where the tachyarrhythmia includes intervals in both interval ranges is reduced by adjusting the interval ranges as a function of the relative distribution of measured intervals within the interval ranges.

This is a continuation of application Ser. No. 08/157,360 filed on Nov.23, 1993, now U.S. Pat. No. 5,403,352.

BACKGROUND OF THE INVENTION

This invention relates to devices which detect and/or treattachyarrhythmias (rapid heart rhythms), and more specifically, tomechanisms to distinguish among various tachyarrhythmias and to provideappropriate therapies to treat the identified tachyarrhythmias.

Early automatic tachyarrhythmia detection systems for automaticcardioverter/defibrillators relied upon the presence or absence ofelectrical and mechanical heart activity (such as intra-myocardialpressure, blood pressure, impedance, stroke volume or heart movement)and/or the rate of the electrocardiogram to detect hemodynamicallycompromising ventricular tachycardia or fibrillation.

In pacemaker/cardioverter/defibrillators presently in clinicalevaluation, fibrillation is distinguished from ventricular tachycardiausing rate based criteria, In such devices, it is common to specify therate or interval ranges that characterize a tachyarrhythmia as opposedto fibrillation. However, some patients may suffer from ventriculartachycardia and ventricular fibrillation which have similar oroverlapping rates, making it difficult to distinguish low ratefibrillation from high rate tachycardia. In addition, ventricularfibrillation may display R--R intervals which may vary considerably,resulting in intervals that may fall within both the tachycardia andfibrillation rate or interval ranges, or outside both.

Presently available pacemaker/cardioverter/defibrillator arrhythmiacontrol devices, such as the Model 7216A and 7217Bpacemaker/cardioverter/defibrillator devices available from Medtronic,Inc., employ programmable fibrillation interval ranges and tachycardiadetection interval ranges which are adjacent to one another but do notoverlap. In these Medtronic devices in particular, the interval rangedesignated as indicative of fibrillation consists of intervals less thana programmable interval (FDI) and the interval range designated asindicative of ventricular tachycardia consists of intervals less than aprogrammable interval (TDI) and greater than or equal to FDI. R--Rintervals are counted to provide a count of R--R intervals fallingwithin the tachycardia interval range (VTEC) and a count of intervalswithin the fibrillation range (VFEC). VFEC is a count of how many of thepreceding series of a predetermined number (FEB) of R--R intervals isless than or equal to FDI. The VTEC count is incremented in response toR--R intervals that are greater than or equal to FDI but shorter thanTDI, is reset to zero in response to intervals longer than or equal toTDI and is insensitive to intervals less than FDI. VTEC is compared to aprogrammed value (VTNID) and VFEC is compared to a correspondingprogrammable value (VFNID). When one of the counts equals itscorresponding programmable value, the device diagnoses the presence ofthe corresponding arrhythmia, i.e., fibrillation or tachycardia anddelivers an appropriate therapy, e.g., anti-tachycardia pacing, acardioversion pulse or a defibrillation pulse. In addition, thephysician may optionally program the device to require that measuredR--R intervals meet a rapid onset criterion before the VTEC count can beincremented and may also optionally program the device to require that arate stability criterion be met with each successive measured R--Rinterval in order to increment VTEC and that otherwise VTEC will bereset to zero. This detection system has proven effective indistinguishing between fibrillation and ventricular tachycardia so thatappropriate therapies may be delivered. However, in rare instances, thedetection methodology may require a sequence of a greater number ofrapid heart beats than might optimally be desired to determine whetherthe rapid rhythm is due to fibrillation or tachycardia. Moreover, animproved level of accuracy in classifying rhythms close to FDI inaverage R--R interval duration is also believed desirable.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide for accuratedetection of and discrimination between tachycardia and fibrillation. Itis a further object of the present invention to provide for detection ofthese arrhythmias in as few heartbeats as possible, consistent withaccurate detection.

In accordance with the present invention, it is realized that because ofthe randomness of intervals between depolarizations during fibrillationor because of the characteristics of a patient's heart rhythms,fibrillation and tachycardia may include such intervals of similarduration. Thus, in a device which defines interval or rate ranges asindicative of fibrillation and tachycardia, intervals falling within therange defined as indicative of fibrillation may in fact be occurringduring tachycardia, and vice versa. Moreover, either tachycardia orfibrillation may include intervals in both rate or interval ranges,delaying detection and identification of the arrhythmia.

The present invention addresses these problems by increasing ordecreasing the duration of the minimum interval indicative offibrillation and/or the maximum interval indicative of tachycardia(which, in the disclosed embodiment are essentially the same value) as afunction of the measured cycle lengths in a sample of the "N" mostrecent intervals between depolarizations, that fall within the intervalranges indicative of tachycardia or fibrillation. During the detectionof a rhythm containing a series of fast such intervals, the inventiondetermines whether the number of intervals within the interval rangeindicative of fibrillation exceeds a predetermined number or percentage.If so, the value of the minimum interval indicative of fibrillation isincremented. The invention may also or alternatively determine whetherthe number of such intervals within the interval range indicative oftachycardia exceeds a predetermined number or percentage. If so, thevalue of the minimum interval indicative of fibrillation is decremented.Thus, the detection criteria are biased toward detection of fibrillationor tachycardia depending on the relative numbers of measured intervalsfalling within the associated rate zones. This bias increases the speedof detection by making it more likely that detected intervals willassist in meeting the detection criteria for one of these arrhythmiasthan the other. In rhythms with average intervals betweendepolarizations near the minimum interval duration which defines thedividing line between the tachycardia and fibrillation interval ranges,the invention in general reduces the total number of intervals requiredto satisfy the detection criteria for fibrillation or tachycardia.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and still further objects, features and advantages of thepresent invention will become apparent from the following detaileddescription of a presently preferred embodiment, taken in conjunctionwith the accompanying drawings, and, in which:

FIG. 1 is a diagram illustrating the interval ranges employed fordetection of tachyarrhythmias in the disclosed embodiment of the presentinvention.

FIG. 2 is a simplified block diagram illustrating the components of adevice within which the method and apparatus of the present inventionmay be implemented.

FIGS. 3a and 3b show a simplified flow chart diagram illustrating thefunctioning of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the embodiment of the invention discussed below, the tachycardia andfibrillation detection criteria discussed above in conjunction with theMedtronic Model 7216 and Model 7217 implantablepacemaker/cardioverter/defibrillators are employed, and the followingdiscussion of the present invention should be understood in thiscontext, with the present invention being employed in this embodiment toadjust the dividing point FDI between the interval ranges associatedwith ventricular tachycardia and ventricular fibrillation. However,while the specific embodiment disclosed below is directed todistinguishing between ventricular tachycardia and fibrillation, it isalso believed that the invention may also be usefully be practiced inthe context of a device for treating atrial tachyarrhythmias. Moreover,the value of the present invention is not limited to the context of thespecific detection criteria disclosed, but is believed workable andvaluable in the context of any devices which distinguish betweentachycardia and fibrillation using rate or interval based criteria.

FIG. 1 illustrates the relationship between the interval rangesassociated with detection of fibrillation and tachycardia, as employedin the context of the present invention. The maximum interval indicativeof fibrillation (minimum interval indicative of tachycardia), "FDI_(p) "is defined during programming of the device. The maximum intervalindicative of tachycardia, TDI, is similarly defined during programmingof the device. As illustrated, an increments to (FDI_(p) plus delta) ordecrements from (FDI_(p) minus delta) the value the value of FDI_(p) mayoccur as part of the detection process including the present invention.Initially, FDI_(p) serves as the current maximum interval indicative offibrillation (FDI_(c)). As increments or decrements are made, theincremented or decremented values are used as FDI_(c). Increments areonly allowable up to the point where the incremented value would exceeda maximum value (FDI_(max)). Decrements are only allowable up to thepoint where the decremented value would be less than a minimum value(FDI_(min)).

In the context of the preferred embodiment of the present invention,four or eight preceding R--R intervals less than TDI, for example, maybe examined to determine whether the duration of FDI_(c) needs to beincremented or decremented. The value of the increment delta may be, forexample, 10 to 30 ms, and the values of FDI_(max) and FDI_(min) may be,for example, FDI_(c) plus 20 to 60 milliseconds and FDI_(c) minus 20 to60 milliseconds, respectively. Alternatively, the invention may bepracticed in a fashion such that FDI_(p) defines either the maximumvalue or the minimum value of FDI_(c). For example, the physician maywish to allow the device only to become more biased toward detection ofventricular fibrillation, as compared to detection using the programmedrate interval ranges. In this case, FDI_(min) would be set equal toFDI_(p).

FDI_(c) may be incremented, for example, in response to more than fiftypercent or more of the N intervals being less than FDI_(c) and FDI_(c)may correspondingly be decremented in response to more than fiftypercent of the N intervals being greater than or equal to FDI_(c).Alternatively, more stringent criteria for incrementing and decrementingFDI_(c) may be applied, with incrementing occurring only in response toseventy-five percent or more of the N intervals being less than FDI_(c)and decrementing occurring only in response to seventy-five percent ormore of the N intervals being greater than or equal to FDI_(c). It isanticipated that in greater than or equal to FDI_(c). It is anticipatedthat in commercial embodiments of the present invention, some or all ofthe values and parameters discussed above will be selectable by thephysician.

FIG. 2 is a functional schematic diagram of an implantablepacemaker/cardioverter/defibrillator in which the present invention mayusefully be practiced. This diagram should be taken as exemplary of thetype of device in which the invention may be embodied, and not aslimiting, as it is believed that the invention may usefully be practicedin a wide variety of device implementations, including devices havingfunctional organization similar to any of the implantablepacemaker/defibrillator/cardioverters presently being implanted forclinical evaluation in the United States. The invention is also believedpracticable in conjunction with implantablepacemaker/cardioverters/defibrillators as disclosed in prior U.S. Pat.No. 4,548,209, issued to Wielders, et al. on Oct. 22, 1985, U.S. Pat.No. 4,693,253, issued to Adams et al. on Sep. 15, 1987, U.S. Pat. No.4,830,006, issued to Haluska et al. on May 6, 1989 and U.S. Pat. No.4,949,719, issued to Pless et al. on Aug. 21, 1990, all of which areincorporated herein by reference in their entireties.

The device is illustrated as being provided with six electrodes, 500,502, 504, 506, 508 and 510. Electrodes 500 and 502 may be a pair ofendocardial electrodes located in the ventricle, mounted to atransvenous lead. Electrode 504 may correspond to a remote, indifferentelectrode located on the housing of the implantablepacemaker/cardioverter/defibrillator. Electrodes 506, 508 and 510 maycorrespond to the large surface area defibrillation electrodes locatedon ventricular, coronary sinus, superior vena cava or subcutaneous leadsor to epicardial defibrillation electrodes.

Electrodes 500 and 502 are shown as hard-wired to the 514, autothreshold circuit 516 for providing an adjustable sensing threshold as afunction of the measured R-wave amplitude and comparator 518. A signalis generated on R-out line 564 whenever the signal sensed betweenelectrodes 500 and 502 exceeds the present sensing threshold defined byauto threshold circuit 516. As illustrated, the gain on the band passamplifier 514 is also adjustable by means of a signal from the pacertiming and control circuitry 520 on GAIN ADJ line 566.

The operation of this R-wave detection circuitry may correspond to thatdisclosed in commonly assigned, copending U.S. patent application Ser.No. 07/612,760, by Keimel, et al., filed Nov. 15, for an Apparatus forMonitoring Electrical Physiologic Signals, incorporated herein byreference in its entirety. However, alternative R-wave detectioncircuitry such as that illustrated in U.S. Pat. No. 4,819,643, issued toMenken on Apr. 11, 1989 and U.S. Pat. No. 4,880,004, issued to Baker etal. on Nov. 14, 1989, both incorporated herein by reference in theirentireties, may also usefully be employed to practice the presentinvention.

The threshold adjustment circuit 516 sets a threshold corresponding to apredetermined percentage of the amplitude of a sensed R-wave, whichthreshold decays to a minimum threshold level over a period of less thanthree seconds thereafter, similar to the automatic sensing thresholdcircuitry illustrated in the article "Reliable R-Wave Detection fromAmbulatory Subjects", by Thakor et al., published in Biomedical ScienceInstrumentation, Vol. 4, pp 67-72, 1978, incorporated herein byreference in its entirety.

It is preferable that .the threshold level not be adjusted in responseto paced R-waves, but instead should continue to approach the minimumthreshold level following paced R-waves to enhance sensing of low levelspontaneous R-paced R-waves to enhance sensing of low level spontaneousR-waves associated with tachyarrhythmias. The time constant of thethreshold circuit is also preferably sufficiently short so that minimumsensing threshold may be reached within 1-3 seconds following adjustmentof the sensing threshold equal to 70-80% of the amplitude of a detectedspontaneous R-wave. The invention may also be practiced in conjunctionwith more traditional R-wave sensors of the type comprising a band passamplifier and a comparator circuit to determine when the band-passedsignal exceeds a predetermined, fixed sensing threshold.

Switch matrix 512 is used to select which of the available electrodesare coupled to band pass amplifier 534. Selection of which twoelectrodes are so coupled is controlled by the microprocessor 524 viadata/address bus 540. Signals from the selected electrodes are passedthrough band-pass amplifier 534 and into multiplexer 532, where they areconverted to multi-bit digital signals by A/D converter 530, for storagein random access memory 526 under control of direct memory addresscircuit 528.

Microprocessor 524 analyzes the digitized EGM signal stored in randomaccess memory 526 to determine the width of the stored R-wave or inconjunction with the tachycardia/fibrillation discrimination functiondiscussed below.

Amplifier 534 may be a broad band pass amplifier, having a band passextending for approximately 0.5 to 200 hertz. The filtered EGM signalfrom amplifier 534 is passed through multiplexer 532, and digitized inA-D converter circuitry 530. The digitized EGM data is stored in randomaccess memory 526 under control of direct memory address circuitry 528.Preferably, a portion of random access memory 526 is configured as alooping or buffer memory which stores at least the preceding severalseconds of the EGM signal.

The data stored in the buffer memory may be optionally employed toperform R-wave width measurements as disclosed in co-pending U.S. patentapplication Ser. No. 07/867,931, filed Apr. 13, 1992 by Mader et al.,now U.S. Pat. No. 5,312,441, incorporated herein by reference in itsentirety and/or to perform the ventricular fibrillation/ventriculartachycardia discrimination function disclosed in pending U.S. patentapplication Ser. No. 07/750,679 filed Aug. 27, 1991 by Bardy et al., nowU.S. Pat. No. 5,193,535, also incorporated herein by reference in itsentirety. However, the present invention is readily practiced in deviceswhich do not include such functions, and for purposes of the disclosedpreferred embodiment of the present invention it should be assumed thatsuch functions, if available, are programmed off.

The microprocessor also updates counts related to the R--R intervalspreviously sensed. The counts are incremented on the occurrence of ameasured R--R intervals falling within associated rate ranges. Asdiscussed above these ranges may include the ranges illustrated above inFIG. 1 associated with ventricular tachycardia and ventricularfibrillation, and the stored counts may include VTEC and VFEC. Theserate ranges may be defined by the programming stored in the RAM 526.

These counts, along with other stored information reflective of theprevious series of R--R intervals such as information regarding therapidity of onset of the detected short R--R intervals, the stability ofthe detected R--R intervals, the duration of continued detection ofshort R--R intervals, the average R--R interval duration and informationderived from analysis of stored EGM segments are used to determinewhether tachyarrhythmias are present and to distinguish betweendifferent types of tachyarrhythmias, as discussed above in conjunctionwith FIG. 1. Other such detection algorithms for recognizingtachycardias are described in the above cited U.S. Pat. No. 4,726,380,issued to Vollmann, U.S. Pat. No. 4,880,005, issued to Pless et al. andU.S. Pat. No. 4,830,006, issued to Haluska et al., incorporatedby-reference in their entireties herein. An additional set oftachycardia recognition methodologies is disclosed in the article "Onsetand Stability for Ventricular Tachyarrhythmia Detection in anImplantable Pacer-Cardioverter-Defibrillator" by Olson et al., publishedin Computers in Cardiology, Oct. 7-10, 1986, IEEE Computer SocietyPress, pp. 167-170, also incorporated by reference in its entiretyherein. However, other criteria may also be measured and employed inconjunction with the present invention.

It is envisioned that onset and stability requirements are optional in adevice employing the present invention, and preferably are madeavailable as programmable options, which may be deleted by externalprogrammer command. If included, it is believed preferable that theonset criteria be required to met prior to initiating counting of VTEC,and that once met, the criteria will remain satisfied until detection oftachycardia termination. Thus, onset is not intended to be a detectioncriteria required for redetection of tachycardia, following initialdetection. The width criterion, if used, should also be understood to beused both in initial detection of tachycardia and in redetection oftachycardia. This reflects a presumption that following initialdetection of ventricular tachycardia, absent a proven return to normalheart rhythm (termination detect), subsequent high ventricular ratesshould be presumed to be ventricular in origin. The stability criterion,on the other hand, is believed to be appropriate for use both in initialdetection of tachycardia and in redetection of tachycardia.

The remainder of the circuitry is dedicated to the provision of cardiacpacing, cardioversion and defibrillation therapies. The pacertiming/control circuitry 520 includes programmable digital counterswhich control the basic time intervals associated with VVI mode cardiacpacing, including the pacing escape intervals, the refractory periodsduring which sensed R-waves are ineffective to restart timing of theescape intervals and the pulse width of the pacing pulses. The durationsof these intervals are determined by microprocessor 524, and arecommunicated to the pacing circuitry 520 via address/data bus 540. Pacertiming/control circuitry also determines the amplitude of the cardiacpacing pulses and the gain of band-pass amplifier, under control ofmicroprocessor 524.

During VVI mode pacing, the escape interval counter within pacertiming/control circuitry 520 is reset upon sensing of an R-wave asindicated by a signal on line 564, and on timeout triggers generation ofa pacing pulse by pacer output circuitry 522, which is coupled toelectrodes 500 and 502. The escape interval counter is also reset ongeneration of a pacing pulse, and thereby controls the basic timing ofcardiac pacing functions, including antitachycardia pacing. The durationof the interval defined by the escape interval timer is determined bymicroprocessor 524, via data/address bus 540. The value of the countpresent in the escape interval counter when reset by sensed R-waves maybe used to measure the duration of R--R intervals, to detect thepresence of tachycardia and to determine whether the minimum ratecriteria are met for activation of the width measurement function.

Microprocessor 524 operates as an interrupt driven device, and respondsto interrupts from pacer timing/control circuitry 520 corresponding tothe occurrence of sensed R-waves and corresponding to the generation ofcardiac pacing pulses. These interrupts are provided via data/addressbus 540. Any necessary mathematical calculations to be performed bymicroprocessor 524 and any updating of the values or intervalscontrolled by pacer timing/control circuitry 520 take place followingsuch interrupts.

In the event that a tachyarrhythmia is detected, and ananti-tachyarrhythmia pacing regimen is desired, appropriate timingintervals for controlling generation of antitachycardia pacing therapiesare loaded from microprocessor 524 into the pacer timing and controlcircuitry 520, to control the operation of the escape interval counterand to define refractory periods during which detection of an R-wave bythe R-wave detection circuitry is ineffective to restart the escapeinterval counter. Similarly, in the event that generation of acardioversion or defibrillation pulse is required, microprocessor 524employs the counters to in timing and control circuitry 520 to controltiming of such cardioversion and defibrillation pulses, as well astiming of associated refractory periods during which sensed R-waves areineffective to reset the timing circuitry.

In response to the detection of fibrillation or a tachycardia requiringa cardioversion pulse, microprocessor 524 activatescardioversion/defibrillation control circuitry 554, which initiatescharging of the high voltage capacitors 556, 558, 560 and 562 viacharging circuit 550, under control of high voltage charging line 552.The voltage on the high voltage capacitors is monitored via VCAP line538, which is passed through multiplexer 532, and, in response toreaching a predetermined value set by microprocessor 524, results ingeneration of a logic signal on CAP FULL line 542, terminating charging.Thereafter, delivery of the timing of the defibrillation orcardioversion pulse is controlled by pacer timing/control circuitry 520.One embodiment of an appropriate system for delivery and synchronizationof cardioversion and defibrillation pulses, and controlling the timing.functions related to them is disclosed in more detail in co-pending,commonly assigned U.S. patent application Ser. No. 07/612,761, byKeimel, for an Apparatus for Detecting and Treating a Tachyarrhythmia,filed Nov. 15, 1990 and incorporated herein by reference in itsentirety. However, any known cardioversion or defibrillation pulsegeneration circuitry is believed usable in conjunction with the presentinvention. For example, circuitry controlling the timing and generationof cardioversion and defibrillation pulses as disclosed in U.S. Pat. No.4,384,585, issued to Zipes on May 24, 1983, in U.S. Pat. No. 4,949,719issued to Pless et al., cited above, and in U.S. Pat. No. 4,375,817,issued to Engle et al., all incorporated herein by reference in theirentireties may also be employed. Similarly, known circuitry forcontrolling the timing and generation of antitachycardia pacing pulsesas described in U.S. Pat. No. 4,577,633, issued to Berkovits et al. onMar. 25, 1986, U.S. Pat. No. 4,880,005, issued to Pless et al. on Nov.14, 1989, U.S. Pat. No. 4,726,380, issued to Vollmann et al. on Feb. 23,1988 and U.S. Pat. No. 4,587,970, issued to Holley et al. on May 13,1986, all of which are incorporated herein by reference in theirentireties may also be used.

In modern pacemaker/cardioverter/defibrillators, the particularanti-tachycardia and defibrillation therapies are programmed into thedevice ahead of time by the physician, and a menu of therapies istypically provided. For example, on initial detection of tachycardia, ananti-tachycardia pacing therapy may be selected. On re-detection oftachycardia, a more aggressive anti-tachycardia pacing therapy may bescheduled. If repeated attempts at antitachycardia pacing therapiesfail, a higher level cardioversion pulse therapy may be selectedthereafter. Prior art patents illustrating such pre-set therapy menus ofanti-tachyarrhythmia therapies include the above-cited U.S. Pat. No.4,830,006, issued to Haluska, et al., U.S. Pat. No. 4,727,380, issued toVollmann et al. and U.S. Pat. No. 4,587,970, issued to Holley et al. Thepresent invention is believed practicable in conjunction with any of theknown anti-tachycardia pacing and cardioversion therapies, and it isbelieved most likely that the invention of the present application willbe practiced in conjunction with a device in which the choice and orderof delivered therapies is programmable by the physician, as in currentimplantable pacemaker/cardioverter/defibrillators.

In addition to varying the therapy delivered following a failed attemptto terminate a tachyarrhythmia, it is also known that adjustment ofdetection criteria may be appropriate. For example, adjustment maycomprise reducing the number of intervals required to detect atachyarrhythmia to allow a more rapid re-detection or by changing theinterval ranges to bias detection towards detection of ventricularfibrillation, for example as disclosed in U.S. Pat. No. 4,971,058,issued to Pless et al. and incorporated herein by reference in itsentirety.

In the present invention, selection of the particular electrodeconfiguration for delivery of the cardioversion or defibrillation pulsesis controlled via output circuit 548, under control ofcardioversion/defibrillation control circuitry 554 via control bus 546.Output circuit 548 determines which of the high voltage electrodes 506,508 and 510 will be employed in delivering the defibrillation orcardioversion pulse regimen, and may also be used to specify amulti-electrode, simultaneous pulse regimen or a multi-electrodesequential pulse regimen. Monophasic or biphasic pulses may begenerated. One example of circuitry which may be used to perform thisfunction is set forth in commonly assigned co-pending patent applicationSer. No. 07/612,758, filed by Keimel, for an Apparatus for DeliveringSingle and Multiple Cardioversion and Defibrillation Pulses, filed Nov.14, 1990, incorporated herein by reference in its entirety. However,output control circuitry as disclosed in U.S. Pat. No. 4,953,551, issuedto Mehra et al. on Sep. 4, 1990 or U.S. Pat. No. 4,800,883, issued toWinstrom on Jan. 31, 1989 both incorporated herein by reference in theirentireties, may also be used in the context of the present invention.Alternatively single monophasic pulse regimens employing only a singleelectrode pair according to any of the above cited references whichdisclose implantable cardioverters or defibrillators may also be used.

FIGS. 3a and 3b illustrate the function of the present invention asembodied in a device as illustrated in FIG. 2, in the form of flowcharts. FIG. 3b illustrates the FDI ADJUST functional block 11 of FIG.3a in more detail.

FIG. 3a illustrates the overall tachyarrhythmia detection function asemployed in the disclosed embodiment of the present invention. With theexception of functional block 11, this portion of the tachyarrhythmiadetection function corresponds to that employed in the Medtronic Model7216 and 7217 implantable pacemaker/cardioverter/defibrillators,discussed above. In the context of FIG. 3a, the device should beunderstood to be operating as a demand pacer, with the detectionfunctions illustrated taking place during the refractory periodfollowing the occurrence of a spontaneous or paced R-wave.

The microprocessor waits at 10 for an interrupt indicating theoccurrence of a paced or sensed R-wave and in response thereto storesthe duration of the preceding R--R interval and increments the value ofVFEC or VTEC, if appropriate, using the interval criteria discussedabove, based on the value of FDI_(c) and TDI. The microprocessor thendetermines whether the value of FDI_(c) needs to be incremented ordecremented at 11. At 12, the microprocessor determines whether thedetection criteria for ventricular fibrillation have been met, i.e.,whether VFEC is greater than or equal to VFNID. If ventricularfibrillation is detected, then the scheduled defibrillation therapy isinitiated in block 16 and the detection criteria and therapy menus areupdated at 18, as described above.

If ventricular fibrillation is not detected at 12, the microprocessorchecks at 14 to determine whether the criteria for detection ofventricular tachycardia have been met, i.e. whether VTEC is greater thanor equal to VTNID. If ventricular tachycardia is detected, the scheduledventricular tachycardia therapy is delivered at 16 and the detectioncriteria and therapy menus are updated at 18, as described above.

If no tachyarrhythmia is detected, if a tachycardia was previouslydetected, the microprocessor checks at 22 to determine whether a returnto sinus rhythm has occurred, i.e. a series of a predetermined number ofR--R intervals greater than or equal to TDI. If termination is detected,the detection criteria and therapy menus are updated at 24, as describedabove.

The flow chart of FIG. 3b illustrates the method by which the value ofFDI_(c) is adjusted in block 11 of FIG. 3a, based on the proportion ofthe N most recent event intervals that are shorter than the FDI_(c)value. Decision block 40 determines whether or not N event intervalsless than TDI have been stored since the last updating of the detectioncriteria due to detection of a tachyarrhythmia or detection oftermination of a tachyarrhythmia. If not, the adjustment function is notenabled, and the microprocessor continues with the detection methodologyof FIG. 3a. N is intended to be a relatively small number, e.g. four oreight, preferably less than VFNID or VTNID, so detection of fibrillationor tachycardia, as a practical matter will not occur until there has atleast been an opportunity for the adjustment function to operate.

If N intervals less than TDI have been detected, decision block 42determines whether or not a predetermined number of the N intervals areless than the value of FDI_(c). If a predetermined number M of the ofthe N stored event intervals are less than FDI_(c), for example three ormore of four, the microprocessor will check at 44 to determine whetherFDI_(c) is already at its maximum value. If it is at its maximum value,then it is not altered, and the present value of FDI_(c) is employed inthe detection methodology of FIG. 3a. However, if FDI_(c) is not yet atits programmed maximum value, then FDI_(c) is incremented by delta inblock 46.

If M of the of the N stored event intervals are not less than FDI_(c),the microprocessor will check at 43 to determine whether P of the of theN stored intervals are greater than or equal to FDI_(c), for examplethree or more of four.

If not, the adjustment function is not enabled, and the microprocessorcontinues with the detection methodology of FIG. 3a. If P of the Nintervals are greater than or equal to FDI_(c), the microprocessorchecks at 48 to determine whether FDI_(c) is already at its minimumvalue. If it is at its minimum value, then it is not altered, and thepresent value of FDI_(c) is employed in the detection methodology ofFIG. 3a. However, if FDI_(c) is not yet at its programmed minimum value,then FDI_(c) is decremented by delta in block 50.

Through these adjustments of the FDI_(c), the detection functionillustrated in FIG. #a is rendered more sensitive to the trend of the Nmost recent event intervals. It is thus expected, that in practice theadjustment of the FDI_(c) will prove beneficial in accelerating adetection of ventricular tachycardia or ventricular fibrillation inthose cases in which the R--R intervals of the patient's tachyarrhythmiainclude intervals greater and less than FDI.

After incrementing or decrementing the value of FDI_(c), the new valueis used in subsequent detection functions illustrated in FIG. 3a. Thenew value of FDI_(c) may be employed in various ways. The simplestmanner in which the new value of FDI_(c) may be employed is forsubsequent R--R intervals to be classified based on the new value, withthe classification of preceding R--R intervals left undisturbed. In thiscase, the vales of VFEC and VTEC would remain unaltered as a result ofthe adjustment function, and would simply be subsequently incrementedusing the new interval ranges defined using the adjusted value ofFDI_(c).

An alternative method of employing the adjusted value of FDI_(c) is toapply the new interval ranges defined by the adjusted value bothprospectively and retrospectively. In this case, previously stored R--Rintervals would be reexamined, and the VFEC and VTEC counts updated tothe values which they would have had if the adjusted value of FDI_(c)had been in effect throughout the detection sequence.

Following a completed detection sequence or detection of termination ofa previously detected tachyarrhythmia, the value of FDI_(c) is reset tobe equal to FDI_(p), as part of the procedure for updating the detectioncriteria in functional blocks 18 and 24 in FIG. 3a. In some embodimentsof the invention, the value of FDI_(p) may be different duringredetection sequences from the value during initial detection sequences.In such cases, during re-detection sequences, the adjustment functionmay be employed using FDI_(c) set to the current value of FDI_(p).Alternatively, in such embodiments and in other embodiments of theinvention, the adjustment function may be dispensed with entirely duringre-detection sequences.

While the preferred embodiment of the device takes the form of amicroprocessor controlled device as illustrated in FIG. 2, in which thevarious functional steps illustrated in FIGS. 3a and 3b would beimplemented in the form of software, the invention may equally well bepracticed in the form of a dedicated, full custom digital integratedcircuit or, even in the form of an analog circuit, employing analogvalues as substitutes for the digital values disclosed in conjunctionwith the above specification.

In addition, while the preferred embodiment disclosed above takes theform of a pacemaker/cardioverter/defibrillator, the enhanced ability todistinguish between various tachyarrhythmias and the improved speed ofdetection provided by the present invention are also valuable andapplicable to devices which are only capable of performing a subset ofthe various therapies discussed above in conjunction with FIG. 2. Forexample, the ability to accurately distinguish between ventriculartachycardia and ventricular fibrillation would be valuable in anantitachycardia pacemaker, even without a cardioversion pulse generator,to determine whether anti-tachycardia pacing therapies are appropriate.Similarly, the ability to distinguish between ventricular tachycardiaand ventricular fibrillation is valuable in an implantablecardioverter/defibrillator lacking a cardiac pacing function, forexample, as in the currently available CPI AICD implantablecardioverter/defibrillators. It should further be kept in mind thatwhile the therapies described for delivery in response to detection ofthe various arrhythmias discussed are all disclosed in the context ofelectrical therapies, it is possible that the invention may be embodiedin the form of an implantable drug dispenser, wherein one or more of theanti-tachycardia therapies takes the form of injection of a drug locallyinto the heart or systemically to treat the detected arrhythmia. Assuch, the above disclosure should be taken merely as an example of anembodiment of the present invention, rather than limiting, when readingthe claims which follow.

In conjunction with the above specification, I claim:
 1. An apparatusfor detection and treatment of tachyarrhythmias, comprising:means forsensing depolarizations of a patient's heart; means for measuringintervals separating successive depolarizations of said patient's heart;means for defining a first interval range extending between a firstinterval duration and a second interval duration less than said firstinterval duration and a second interval range including intervaldurations less than said second interval duration; means for determiningwhether said measured intervals fall within said first or secondinterval ranges; first diagnosing means for diagnosing occurrence of afirst type of arrhythmia in response to said measured intervals fallingwithin said first interval range; second diagnosing means for diagnosingoccurrence of a second type of arrhythmia in response to said measuredintervals falling within said second interval range; interval rangeadjustment means responsive to said determining means for adjusting saidsecond interval duration, for subsequent use by said determining means,as a function of the durations of said measured intervals preceding adiagnosis of said first or second type of arrhythmia; means fordelivering a first anti-arrhythmia therapy in response to detection ofsaid first type of arrhythmia; and means for delivering a secondanti-arrhythmia therapy in response to detection of said second type ofarrhythmia.
 2. An apparatus according to claim 1 wherein said adjustmentmeans comprises means for determining numbers of said measured intervalsfalling within said first and second interval ranges.
 3. An apparatusaccording to claim 2 wherein said adjusting means comprises means forincreasing said second interval duration in response to the number ofsaid measured intervals falling within said second interval rangeequalling or exceeding a predetermined first value.
 4. An apparatusaccording to claim 2 wherein said adjusting means comprises means fordecreasing said second interval duration in response to the number ofsaid measured intervals falling within said first interval rangeequalling or exceeding a predetermined second value.
 5. An apparatusaccording to claim 3 or claim 4 wherein said first and secondpredetermined values comprise predetermined numbers of said measuredintervals falling within said first and second interval ranges,respectively.
 6. An apparatus according to claim 1 or claim 2 whereinsaid first diagnosing means comprises means for diagnosing theoccurrence of a tachycardia and wherein said second diagnosing meanscomprises means for diagnosing the occurrence of fibrillation.
 7. Anapparatus according to claim 1 or claim 2 wherein said means for sensingdepolarizations comprises means for sensing depolarizations of saidpatient's ventricle.
 8. A method of detection and treatment oftachyarrhythmias, comprising:sensing depolarizations of a patient'sheart; measuring intervals separating successive depolarizations of saidpatient's heart; defining a first interval range extending between afirst interval duration and a second interval duration less than saidfirst interval duration and a second interval range including intervaldurations less than said second interval duration; determining whethersaid measured intervals fall within said first or second intervalranges; diagnosing occurrence of a first type of arrhythmia in responseto said measured intervals falling within said first interval range;diagnosing occurrence of a second type of arrhythmia in response to saidmeasured intervals falling within said second interval range; adjustingsaid second interval duration as a function of the durations of saidmeasured intervals preceding diagnosis of said first or second type ofarrhythmia; delivering a first anti-arrhythmia therapy in response todetection of said first type of arrhythmia; delivering a secondanti-arrhythmia therapy in response to detection of said second type ofarrhythmia; and subsequently employing said adjusted second intervalduration in determining whether said measured intervals fall within saidfirst or second interval ranges.
 9. A method according to claim 8wherein said adjusting step comprises determining numbers of saidmeasured intervals falling within said first and second interval ranges.10. A method according to claim 9 wherein said adjusting step comprisesincreasing said second interval duration in response to the number ofsaid measured intervals falling within said second interval rangeequalling or exceeding a predetermined first value.
 11. A methodaccording to claim 9 wherein said adjusting step comprises decreasingsaid second interval duration in response to the number of said measuredintervals falling within said first interval range equalling orexceeding a predetermined second value.
 12. A method according to claim10 or claim 11 wherein said step of sensing depolarizations comprisessensing depolarizations of said patient's ventricle.
 13. A methodaccording to claim 8 or claim 9 wherein step of diagnosing said firsttype of arrhythmia comprises diagnosing the occurrence of a tachycardiaand wherein said step of diagnosing said second type of tachyrhythrniacomprises diagnosing the occurrence of fibrillation.